The present invention relates to a recording medium which is capable of realizing a high-density recording by means of light irradiation or electric charge injection, and to a method of manufacturing the recording medium.
It is desired in the latest information-oriented society to develop a recording and reproducing method and an apparatus which enable to achieve, in conformity with an increasing quantity of information, an extremely high recording density as compared with the conventional methods and apparatuses.
There are two kinds of technique for realizing optical recording, i.e., heat mode recording in which light is employed as heat, and photon mode recording in which light is employed as it is without being converted into heat. The heat mode recording is put into practical use in magneto-optical recording or in phase-change recording.
In order to improve the recording density by means of the heat mode recording, a technique employing a near-field scanning optical microscope (NSOM) which enables to form a microspot smaller than the wavelength of light has been proposed. For example, Betzig et al have succeeded to form recording spots having a size of 60 nm in an attempt to realize magneto-optical recording and reproducing of information by irradiating a Co/Pt multilayered film with an output of Ar ion laser through an NSOM probe (Appl. Phys. Lett., 61, 142 (1992)). Further, Hosaka et al have succeeded to form recording spots having a size of 50 nm by irradiating a 30 nm-thick Ge.sub.2 Sb.sub.2 Te.sub.5 film with an output of a semiconductor laser through an NSOM probe so as to cause phase change in the film (Thin Solid Film, 273, 122 (1996); J. Appl. Phys., 79, 8092 (1996)). However, these methods are accompanied with problems that the recording spots are enlarged due to heat diffusion and that they require a large energy. Therefore, it is expected that these methods are not applicable to the realization of a recording density in the order of terabits/cm.sup.2 where a recording spot size is required to be reduced to as small as about 10 nm.
Under the circumstances, the present inventors disclosed a recording medium which is capable of reading fluorescence by utilizing phase change in an organic dye molecule (Japanese Patent Unexamined Publication H7-254153). According to the recording medium, if an organic dye molecule having a low heat conductivity is employed, it is possible to greatly reduce the recording spot size. However, even with this method, it is still difficult to realize a recording density in the order of terabits/cm.sup.2 where a spot size is required to be reduced to about 10 nm. Further, this recording medium is accompanied with problems that, since amorphous regions are brought into contact with a crystal region, the amorphous regions are likely to be crystallized, thus destroying the recorded data. Furthermore, since the phase change medium is constituted by a uniform medium, a difference in signal level between the crystal region and the amorphous region is relatively small, thus leading to increase in noise.
Likewise, a uniform medium for the photon mode recording employing a photochromic compound is also accompanied with the problem of noise.
On the other hand, Japanese Patent Unexamined Publication H8-45122 discloses a recording medium provided with dot-like recording domains having a size of 10 to 100 nm and comprising an organic dye molecule in which recording is performed by injecting an electric charge into the recording domains. Since a single dot constitutes a recording unit in the recording medium, it is possible to improve a recording density. However, this recording medium is accompanied with the problems that it is difficult to form recording dots, each constituting a recording unit, uniformly in size and position, and that the size and the position of the dot thus formed are likely altered with time. In addition, it is difficult to entirely flatten the recording medium.
As described above, no one has succeeded as yet to realize a practical recording medium capable of recording with a very high density in the order of terabits/cm.sup.2.